In recent years the search for offshore deposits of crude oil and natural gas has been moving into progressively deeper waters. In very deep waters it is common practice to conduct drilling operations from floating vessels or platforms. The floating vessel or platform is positioned over the subsea wellsite and is equipped with a drilling rig and associated drilling equipment. To conduct drilling operations from a floating vessel or platform, a large diameter pipe known as a"marine drilling riser" or"drilling riser" is typically employed. The drilling riser extends from above the surface of the body of water downwardly to a wellhead located on the floor of the body of water. The drilling riser serves to guide the drill string into the well and provides a return conduit for circulating drilling fluids (also known as"drilling mud" or simply "mud").
An important function performed by the circulating drilling fluids is well control. The column of drilling fluid contained within the well bore and the drilling riser exerts hydrostatic pressure on the subsurface formation which overcomes formation pore pressure and prevents the influx of formation fluids into the well bore, a condition known as a"kick." However, if the column of drilling fluid exerts excessive hydrostatic pressure, another problem can occur, i.e., the pressure of the drilling fluid can exceed the natural fracture pressure of one or more of the exposed subsurface formations. Should this occur, the hydrostatic pressure of the drilling fluid could initiate and propagate a fracture in the formation, resulting in drilling fluid loss to the formation, a condition known as"lost circulation". Excessive fluid loss to one formation can result in loss of well control in other formations being drilled, which greatly increases the risk of a blowout.
For a conventional offshore drilling system, in which the mud in the well and the drilling riser constitute a continuous fluid column from the bottom of the well to the drilling rig at the surface of the body of water, it is increasingly difficult as water depth increases to maintain the pressure in the borehole intermediate the pore pressure and fracture pressure of the exposed formations. This pressure problem limits the length of allowable open borehole and requires frequent installation of protective casing strings, which in turn, results in longer times and higher costs to drill the well. One solution to this difficulty is to maintain the mud pressure at the wellhead (i.e., at the elevation of the floor of the body of water) approximately equal to that of the surrounding water, thus effectively eliminating the influence of the overlying body of water.
Various methods for accomplishing this objective are known in the art, including mechanically pumping the drilling mud from the seafloor and injection of lower density gases, liquids or solids into the drilling mud to decrease the effective density of the mud column to that of seawater. Since all such methods create the equivalent of a column of seawater in the drilling riser that has a density different from that of the drilling mud in the well, they are known as"dual density" systems. One such system involves injecting a gas ("lift gas") such as nitrogen into the lower end of the drilling riser. When lift gas is injected into the drilling riser, it intermingles with the returning drilling fluid and reduces the equivalent density of the column of drilling fluid in the riser to that of seawater. The column of drilling fluid in the well below the lift gas injection point does not contain lift gas and, accordingly, is denser than the drilling fluid in the riser.
Typically with a gas-lifted drilling riser, the pressure at the top of the riser is maintained at an elevated level (e.g., 250 psig). However, it is often necessary to intermittently shut down the gas lift system and de-pressure the drilling riser, particularly when the returns are lifted in the same conduit which is used to guide drilling tools, casing and other devices from the surface into the subsea well. Although it is practical and desirable to protect the well from variations in riser base pressure (p.sub.rb) during the transition from gas lifting by closing one or more subsea blowout preventors (BOPs), it is important that riser base pressure (p.sub.rb) ultimately return to seawater pressure if any portion of the subsea well is uncased. As described further below, this is particularly true if it will be necessary to open the BOP prior to resuming gas lift, such as is often the case when installing well casing.
One way to achieve the objective of maintaining seawater pressure (p.sub.sw) at the base of the riser when gas lifting is shut down is to cease injection of gas, pump seawater into the base of the riser to replace the gas-lifted mud and de-pressure the riser. This results in the riser being open to the atmosphere at the top and filled with quiescent seawater. This is a desirable condition to create prior to installing casing. The well below the riser remains full of the original, higher density, drilling mud. As casing is inserted into the riser, an equal volume of seawater will be displaced out the top of the riser and the internal pressure at the base of the riser (p.sub.rb) will remain equal to that of the external seawater (p.sub.sw). However, once the bottom of the casing string enters the open well, the higher density drilling mud will be displaced into the bottom of the riser while an equal volume of seawater is displaced out of the top of the riser. Progressively, the seawater in the riser will be replaced by higher density drilling mud and the hydrostatic pressure (p.sub.rb) at the base of the riser and in the well will increase. The increased pressure in the well opposite exposed formations may exceed the fracture pressure of the rock, resulting in the well control problem described above.
One approach to addressing this problem is to open a valve at the base of the drilling riser and let excess drilling mud discharge into the sea. However, this approach presents potential pollution hazards and can be expensive since large volumes of drilling mud will be lost. Another approach is to pump the excess drilling mud back to the surface through a separate conduit. While feasible, this requires a relatively large pump at the seafloor if the mud evacuation rate is high enough to keep pace with the normal rate at which casing is made up and run into the riser and well.
From the foregoing, it can be seen that there is a need for an improved method for installing a well casing into a subsea well being drilled with a gas-lifted or other dual density drilling system. Such a method should be capable of maintaining the riser base pressure (p.sub.rb) relatively constant during the entire process of casing installation. The present invention satisfies this need.